【作者】 张双燕；

【导师】 费本华；

【作者基本信息】 中国林业科学研究院 ， 木材科学与技术， 2011， 博士

【摘要】 木材细胞壁是树木的实质承载结构,是以纤维素微纤丝为增强相,木素/半纤维素为基质的层板结构纳米复合材料。纤维素、半纤维素和木质素各组分在细胞壁中的分布、结合方式以及各组分自身的性能对木材细胞壁及其宏观力学性能有着重要的影响,因此,细胞壁结构和功能研究对了解木材物理力学性质的本质起源,指导树木遗传改良、仿细胞壁结构复合材料设计、以及制浆造纸等应用研究都具有重要意义。本论文以人工林杉木[Cunninghamia lancolata(Lamb)Hook]成熟材晚材管胞为研究对象,采用亚氯酸钠法和碱液浓度分级抽提法选择性地脱除细胞壁中的木质素和半纤维素,应用湿化学分析、红外和拉曼光谱技术监测细胞壁化学成分变化,利用X射线衍射技术分析化学脱除方式对细胞壁晶体结构的影响,并利用单根纤维拉伸技术和纳米压痕技术测试基体成分脱除后细胞壁的力学性能响应,揭示了化学成分对木材细胞壁力学性能的影响机理。论文的主要结论和成果如下:(1)光谱分析表明,亚氯酸钠处理后,细胞壁中木质素发生降解,随后逐步增加碱液浓度抽提处理,木聚糖发生降解,葡甘露聚糖最后降解。其中,10%氢氧化钠溶液处理后,有部分纤维素II生成。X射线衍射分析表明脱木素和脱半纤维素(6%、8%氢氧化钠溶液处理)后,纤维素相对结晶度提高,纤维素晶型没有变化。10%的氢氧化钠溶液处理后,纤维素相对结晶度降低,纤维素晶型由纤维素I变为纤维素II。同时,纤维素晶体尺寸在脱木素处理后增大,脱半纤维素处理后减小。(2)对杉木单根纤维单周期、多周期循环拉伸研究表明,细胞壁化学成分的变化不会改变单根纤维本身的应力应变特性。单根纤维(微纤丝角10°左右)的拉伸应力-应变曲线从初始到断裂都呈现良好的线性关系,多周期循环拉伸测试证实化学成分选择性脱除处理前后单根纤维均呈现明显的粘弹塑性特性。纳米压痕测试也证实化学处理前后细胞壁压入载荷-位移曲线特性未发生变化。(3)组成细胞壁基质的木质素,在干燥状态下,对细胞壁的纵向拉伸弹性模量和压入模量影响程度不大。木质素(减少99%)脱除后,细胞壁纵向弹性模量损失约为5.10%,压入模量损失约为6.53%。而木质素对细胞壁的拉伸强度、断裂伸长率以及硬度影响显著。细胞壁的拉伸强度和断裂伸长率随着木质素含量的减少而增大,细胞壁的硬度却随着木质素的脱除,降低了16.98%。(4)组成细胞壁基质的另一种成分半纤维素,对细胞壁的纵向拉伸弹性模量和压入模量影响显著。半纤维素脱除后,细胞壁的纵向拉伸弹性模量损失约为11.57%。其中木聚糖脱除后,细胞壁的纵向拉伸弹性模量损失约为9.55%;而葡甘露聚糖脱除后,纵向拉伸弹性模量损失仅为2.24%。细胞壁的压入模量随着半纤维素的脱除损失了约9.16%。半纤维素尤其是木聚糖对细胞壁的拉伸强度、断裂伸长率影响显著。细胞壁的拉伸强度和断裂伸长率随着半纤维素的降解而降低。与脱木素处理的细胞壁相比,半纤维素脱除后,细胞壁的拉伸强度降低了32.15%,断裂伸长率减少了21.12%,硬度仅降低了0.87%。(5)单根纤维拉伸和纳米压痕测试结果表明,与未处理样品相比,基质(木质素和半纤维素)脱除后,纤维素对细胞壁的纵向拉伸模量的贡献仍可达84%,对压入模量的贡献约占85%,对拉伸强度的贡献占96%,对断裂伸长率占95%,对硬度的贡献占82%。(6)杉木木材纤维细胞壁化学成分选择性脱除处理前后纤维的断裂形式存在一定的差异。机械剥离纤维和脱木素处理纤维的断口形状多呈斜齿形,断口粗糙,表现出较多“韧性断裂”特性;而脱半纤维素的纤维断口相比较平滑,表现出明显的“脆性断裂”特性。这与脱半纤维素的纤维较低的平均断裂伸长率的测试结果相一致。(7)从力学的角度,建立了细胞壁各组分之间关系的结构模型。模型表明,纤维素是细胞壁的主要结构成分,在细胞壁中充当骨架物质,是细胞壁强度的主要来源。半纤维素(葡甘露聚糖、木聚糖)连接纤维素微纤丝和木质素,其中大部分木聚糖连接着葡甘露聚糖和木质素,葡甘露聚糖与纤维素微纤丝结合紧密,木聚糖与葡甘露聚糖之间的作用力小于纤维素微纤丝与葡甘露聚糖之间的作用力。木聚糖在细胞壁中充当连接纤维素和木质素的界面偶联剂,其存在对维持细胞壁力学的完整性具有重要作用。木质素与木聚糖相连,但这种连接很容易破坏,其存在在一定程度上增强了细胞壁的力学性能。更多还原

【Abstract】 Wood cell wall is a load-bearing unit in trees. It can be regarded as laminated nano-composites that cellulose microfibril as a reinforcement embedded in matrix of hemicellulose and lignin. Wood cell wall mainly consists of cellulose, hemicellulose and lignin. The arrangement of chemical components, their interaction and their mechanical properties results in specific mechanical properties of wood cell wall, which finally affect the macroscopic properties of wood. A better understanding of these structure-property relationships is crucial for insighting into the origin of the nature of wood physical and mechanical properties, for guiding the genetic improvement of trees, for cell wall structural composite material stimulations, as well as for a substantial improvement of wood and paper products. In this study, the mature latewood tracheids of Chinese Fir (Cunninghamia lancolata (Lamb.) Hook) were selected, and targeted modification method, which is sodium chlorite (NaClO2) for delignification and sodium hydroxide (NaOH) at different concentrations for extraction of hemicellulose, was used. We applied wet chemical analysis, Infrared and Raman spectroscopy to monitor the changes of chemical components in cell wall. At the same time, we used X-ray diffraction to study the influence of chemical components on cell wall structure. And we used single-fiber-test and nanoindentation technology to study the effect of chemical components on mechanical properties of wood cell wall. Subsequently, we attempted to establish the quantitative relationship between chemical components and mechanical properties of wood cell wall, to clarify the mechanism behind. The main results are summarized as follows:(1) Spectra analysis showed that the degradation of lignin happened first after treatment with sodium chlorite, thereafter xylan degraded, and finally glucomannan degraded by treatments with sodium hydroxide at different concentrations. At the same time, cellulose I transformed into cellulose II after treatment with 10% NaOH. The crystallinity and cellulose crystallite size of different treatments were studied by X-ray diffraction. And the relative crystallinity increased. While the cellulose crystal structure transformed into cellulose II after treatment with 10% NaOH. Meanwhile the relative crystallinity decreased. The cellulose crystallite size increased after the delignification treatment. However,the cellulose crystallite size reduced after the hemicellulose-extracted treatment.(2) The tensile test and cyclic tensile tests on single fibers of the mature latewood of Chinese Fir showed that the change of chemical components did not affect single fibers tensile behavior. The tensile stress-strain curves of single fibers (MFA around 10°) showed only small plastic deformations before rupture. And the cyclic tensile curves of single fibers with targeted modification treatments also showed visco-elastic plastic characteristic.(3) Lignin as one of the matrix had little impact on the longitudinal tensile elastic modulus and indentation modulus of cell wall in dry condition. The loss rate of longitudinal tensile elastic modulus and indentation modulus was about 5.10% and 6.53% respectively after treatment for delignification (reduced 99%). While lignin affected the tensile strength, elongation and hardness, significantly. The tensile strength and elongation of cell wall increased with the reduction of lignin content. The hardness of cell wall reduced 16.98% after delignified treatment.(4) Hemicelluloses affected the longitudinal tensile elastic modulus and indentation modulus of cell wall, significantly. The loss rate of longitudinal tensile elastic modulus and indentation modulus was about 11.57% and 9.16%, respectively, after hemicellulose extracted treatments; and the loss rate of longitudinal tensile elastic modulus was 9.55% after xylan extracted; while after glucomannan extracted, the loss rate was only 2.24%. Hemicellulose (especially xylan) affected the tensile strength and elongation of cell wall, significantly. The tensile strength and elongation of cell wall increased with the degradation of hemicellulose. Compared to delignified cell wall, the tensile strength,elongation, hardness reduced 32.15%, 21.12%,0.87%, respectively after hemicellulose ectracted.(5) Single fiber tests and nanoindentation tests showed that the contribution of cellulose for longitudinal tensile elastic modulus up to 84%, for indentation modulus up to 85%, for the tensile strength up to 96%, for elongation up to 95% and for hardness up to 82%. (6) The fracture mode of mechanical isolated fiber, delignified fiber and hemicellulose-extracted fiber was different. The fracture shape of mechanical isolated fiber and delignified fiber presented oblique tooth profile and the fracture surface was rough. It showed obvious ductile brittle fracture performance. While the fracture shape of hemicellulose-extracted fiber, had low elongation, brittle surface, and smoother than the other fibers.(7) We established a structural model on the relationship between cellulose, hemicellulose and lignin in cell wall. It showed that cellulose which was the source of cell wall strength, was the main structural component in cell wall working as a framework substance. Hemicelluloses (xylan and glucomannan) connected with the highly ordered cellulose of the microfibrils and lignin, and most of xylan contacted with glucomannan and lignin. Glucomannan and cellulose in close contact within the cell wall, and the force between xylan and glucomannan was less than that between glucomannan and cellulose. Xylan acted as an interfacial coupling agent between highly ordered cellulose of the microfibrils and lignin, which was important to maintain the integrity of cell wall mechanics. Lignin linked to xylan, but the connection was easy to destroy. To a certain extent, its existence enhanced the mechanical properties of cell wall.更多还原